The emergence of trenchless technology has created a necessity for knowing real-time orientation/positional information from an underground drill toolhead. During guided underground boring operations, the toolhead's depth, roll position, and inclination is required in order to navigate the boring tool through the soil, around and under objects.
Prior art inclination sensors utilize several different angle measuring methods, including mechanical (pendulum or "gravity-seeking"), resistive (electrolytic fluid based) capacitive (dielectric fluid based), as well as gravity (accelerometer) based techniques. Typically, prior art inclinometers using mechanical, resistive, capacitive, or accelerometer based sensors are single axis devices and cannot provide inclination information independent of roll position. The inventions described in U.S. Pat. Nos. 4,028,815, 3,442,023, and 4,244,117 are based on either electrolytic fluid or conductive fluid resistive techniques and must be oriented at a specific roll position to function properly. U.S. Pat. No. 4,779,353 and U.K. Pat. No. 2110374A also use fluid-resistive techniques, but are, however, independent of roll. U.S. Pat. Nos. 4,644,662 and 4,521,973 use a capacitive/dielectric fluid based method and are not independent of roll, but U.S. Pat. No. 4,811,491 describes a two-axis capacitive/dielectric fluid based inclinometer which provides both roll and incline information. U.S. Pat. No. 5,128,867 uses both flux magnetometers and triaxial accelerometers to sense the earth's magnetic and gravitational field in order to provide roll and pitch information simultaneously. None of the prior art sensors described above, nor in references cited within, however, use optical/fluid based reflective/refractive methods for transducing inclination information that is independent of the roll position of the sensor.